Environmentally friendly fuels, e.g., alternative fuels to hydrocarbon-based energy sources, are currently of great interest. Of particular interest is hydrogen which is currently the subject of a considerable research effort focused on the various problems and considerations associated with making it commercially competitive with hydrocarbon-based fuels. Hydrogen, however, is volumetrically inefficient to store and transport. For compact storage, hydrogen must be compressed to high pressure and stored in specialized tanks. Hydrogen can be stored in liquid form at very low temperatures, but consuming a portion of the available energy for the liquefaction. In addition, losses of stored liquid hydrogen due to boil off are considerable, and thus it remains an ineffective storage method. As a result, chemical hydrides, particularly sodium borohydride, have been proposed as effective hydrogen storage materials for a variety of applications in both distributed power generation and transportation applications.
The goal in the study of hydrogen storage materials is to develop candidates that possess high gravimetric hydrogen potential. Chemical hydrides, including alkali metal hydrides, alkali metal aluminum hydrides and alkali metal borohydrides, generate hydrogen through a hydrolysis reaction in water resulting in gravimetric hydrogen densities that range from 9 to 25 weight percent of the hydride. When the waters of reaction and solvation are taken into account, the combined hydrolysis systems have gravimetric hydrogen densities that range from 4 to 9 wt. percent. Among the candidate hydrides, sodium borohydride (NaBH4) is considered the leader for many reasons, including safety and convenience. An aqueous solution of sodium borohydride, stabilized with sodium hydroxide at a pH between 11 and 15, has been used as a hydrogen generation fuel.
Sodium borohydride has been proved in prototype testing to deliver hydrogen in a manner and with load characteristics that are essentially non-distinguishable from that of compressed hydrogen. In addition, the fact that it can be stored as an aqueous solution adds to its advantage as a safe and convenient carrier for hydrogen. A limitation on the use of sodium borohydride as a storage material for hydrogen, however, is its solubility which limits storage solutions to 36 wt. % or less at room temperature, with a maximum gravimetric hydrogen storage of 6.6 wt. %. In addition, the discharged fuel stream is a mixture of sodium borate compounds and sodium hydroxide, which is a spectator in the hydrolysis reaction, and is strongly alkaline. There are, however, regulatory and handling issues associated with the transport and storage of both the fuel solution and the discharged solution because of their high pH.
Because of the advantages of sodium borohydride as a hydrogen storage material, a considerable amount of research has been directed to improving the synthesis thereof over the typical industrial processes that are based on either the Schlesinger process (Equation 1) or the Bayer process (Equation 2), both of which are shown below.4NaH+B(OCH3)3→3NaOCH3+NaBH4  (1)Na2B4O7+16Na+8H2+7SiO2→4NaBH4+7Na2SiO3  (2)The Schlesinger process and the Bayer process do not provide a favorable energy balance, because the energy cost of using large amounts of sodium in these reactions is high compared to the energy available from sodium borohydride as a fuel.
A second area of research has focused on the sodium borate byproduct of the use of sodium borohydride as a fuel. In order to gain widespread acceptance, a means must be found to convert the borate byproduct into a useful material, preferably sodium borohydride itself thereby regenerating the fuel. Another aspect of the research concerning the discharged fuel solution is the sodium hydroxide, which must either be recovered and reused or disposed of in an environmentally acceptable manner.
While sodium borohydride remains a primary candidate for an alternative fuel system to fossil fuel, there remains the need for other materials that might serve as hydrogen storage materials for such systems that would possess advantage over sodium borohydride. In accordance with the present invention, it has been found that a metal triborohydride has significant advantages as a hydrogen storage material.
Metal triborohydride salts are known materials. U.S. Pat. No. 3,313,603 claims a number of triborohydride salts and a process for their preparation. The primary interest in these salts to date has been as reducing agents and as intermediates in the synthesis of higher boranes, polyhedral borane anions and transition-metal complexes. U.S. Pat. No. 4,166,843 discloses a method of generating pure hydrogen by combusting a solid propellant formed by adding silicon or aluminum to alkyl-substituted quaternary ammonium octahydrotriboronitride salts. The silicon or aluminum are present to complex available carbon molecules from the combustion that would otherwise react with the hydrogen to form methane. In this way, the combustion of the solid propellant is able to generate pure hydrogen. However, the use of these salts in solid propellants does not suggest that they might be useful as hydrogen storage materials in the fuel compositions of the present invention.
Triborohydride salts are typically produced by the reaction of diborane (B2H6) or a higher borane with sodium metal, sodium amalgam, sodium hydride or sodium borohydride. This synthesis has a significant energy cost hurdle because of the large amounts of sodium required. In addition, the reactions of diborane with sodium or sodium amalgam are slow, typically requiring two days for complete reaction at room temperature. While sodium borohydride is a more reactive species, triborohydride formation only takes place at temperatures in excess of 100° C. at one atmosphere pressure of diborane. While the temperature may be lowered, it is only at the expense of raising the pressure of diborane above one atmosphere.
The reaction of diborane and sodium borohydride may also be conducted at high temperatures in ether, but this reaction entails the expense and handling of the solvent. Other preparations of triborohydride salts include the hydroboration of alkali metal compounds of naphthalene or triphenylboron; the reduction of BH3oTHF by metals, such as potassium, rubidium, cesium and the like; and the reaction of diborane with a metal naphthalide. None of these preparations is advantageous in terms of the potential use of triborohydride salts as hydrogen storage materials.
In accordance with the present invention, it has been found that triborohydride salts are useful as hydrogen storage materials. In order that such use may have practical potential for large scale application, an improved preparation of triborohydride salts is provided that is advantageous over preparations known to date.